A good read:
The idea that Apple will inherit Microsoftâ€™s problems is based in the ignorance that Windowsâ€™ security problems are rooted in its popularity, rather than its poor architectural design. That is not true, as countless examples of viruses attacking minor platforms attest. Malware targets weakness, not popularity. Windows is plagued with malware, not because it is ubiquitous, but because it is riddled with weaknesses.
The Unavoidable Malware Myth, roughlydrafted.com